This invention relates to multi-link telephone intercom systems and, more particularly, to multi-link intercom systems suitable for usage with conventional key telephone sets.
Multi-link telephone intercom systems provide a plurality of intercom links for effecting multiple conversation intercom service between a plurality of telephone stations serviced thereby. Each link constitutes a discrete two-way talking path along which two, three or more stations can communicate when simultaneously accessed thereto. Multi-link intercom systems typically are characterized by a common control which includes a single link selector for switching the subscriber stations to the links. The common control continuously monitors or scans the stations to determine when one subscriber station desires intercom service and, upon sensing or receiving appropriate control signals from a subscriber station indicative that intercom service is desired, the common control switches the link selector to that station and causes appropriate indications to appear at the remaining subscriber stations which indicate that the selector is in use. The link selector then searches for and seizes an available link, whereupon it receives station address information from the station to which it is switched and effects appropriate call indication at another subscriber station which, when taken off-hook, is accessed to the link to complete the talk path. The station desiring intercom service is sometimes referred to hereinafter as the "calling station" and the station at which call indication is effected is sometimes referred to hereinafter as the "called station".
In most practical multi-link telephone intercom systems, therefore, the principal control and link switching functions are concentrated in the common control and specialized equipment is required to provide communication between the subscriber stations and the link selector; however, the station circuits associated with each telephone station are, by comparison with the common control, much simpler and hence less costly. Consequently, these systems tend to be uneconomical in small intercom (e.g. 20 or fewer stations) applications due to the disproportionately high cost of common control versus station costs. Further, since line concentration by space division network, relay matrix or appropriate hard wire connections, is employed to transmit signals, typically in digital format, from the calling station to the common control, special telephone sets must be used at the subscriber stations, each set including only one talk path appearance or one pair of talk path conductors along with a second pair of conductors which act to transmit the digital control signals. These systems therefore cannot be used with conventional key telephone sets in which a dedicated talk path pair is associated with each station appearance and which transmit station address information in the form of tone or dial pulse signals along an accessed intercom talk path.